• Journal of Welding and Joining
• /
• 제5권1호
• /
• pp.34-41
• /
• 1987

To explore the possible application of thermal analysis technique as a probe for finding weld defects, Hydrogen trapping phenomena in Heat Affected Zone (HAZ) of the AISI 5160 spring steel were investigated. HAZ was divided into five parts, which were used as thermal analysis specimens. Two types of trap sites were found in HAZ, ferrite/cementin interface and microvoid. The thermal analysis peak due to the ferrite/cementite interface increased its height toward the weld deposit. The thermal analysis peak due to the microvoid was the highest where the grain size was the smallest. The correspondence between the cold cracking and hydrogen trap nature is also discussed.

• 대한기계학회논문집A
• /
• 제29권11호
• /
• pp.1503-1508
• /
• 2005

Scanning Thermal Microscope (STU) has been known for its superior resolution for local temperature and thermal property measurement. However, commercially available STU probe which is the key component of SThM does not provide resolution enough to explore nanoscale thermal phenomena. Here, we developed a SThM probe fabrication process that can achieve spatial resolution around 50 m. The batch-fabricated probe has a thermocouple junction located at the end of the tip. The size of the thermocouple junction is around 200 m and the distance of the junction from the very end of the tip is 150 m. The probe is currently being used for nanoscale thermal probing of nano-material and nano device.

• 한국전기전자재료학회:학술대회논문집
• /
• 한국전기전자재료학회 1993년도 추계학술대회 논문집
• /
• pp.49-54
• /
• 1993

This paper discusses the reliability of solder joints of electronic devices on printed circuit board. Solder application is usually done by screen printing method for the bonding between outer leads of devices and thick film(Ag/Pd) pattern on Hybrid IC as wel1 as Cu lands on PCB. As result of thermal stresses generated at the solder joints due to the differences of thermal expansion coefficients between packge body and PCB, Micro cracking often occurs due to thermal fatigue failure at solder joints. The initiation and the propagate of solder joint crack depends on the environmental conditions, such as storage temperature and thermal cycling. The principal mechanisms of the cracking pheno- mana are the formation of kirkendal void caused by the differences in diffusion rate of materials, ant the thermal fatigue effect due to the differences of thermal expansion coefficient between package body and PCB. Finally, This paper experimentally shows a way to supress solder joints cracks by using low-${\alpha}$ PCB and the packages with thin lead frame, and investigates the phenomena of diffusion near the bonding interfaces.

• Tribology and Lubricants
• /
• 제14권1호
• /
• pp.70-78
• /
• 1998

Two different friction materials (organic and low-metallic pads) for automotive brakes were studied to investigate the anti-fading phenomena during stop. The anti-fading phenomena were pronounced more in the case of using low metallic friction materials than organic friction materials. The main cause of the anti-fading phenomena was the high dependence of friction coefficient on a sliding speed. The anti-fading was prominent when the initial brake temperature was high in the case of low-metallic friction materials due to the strong stick-slip event at high temperature. On the other hand, the anti-fading was not severe in organic friction materials and the effect was reduced at high braking temperature due to the thermal decomposition of organic friction materials. The strong stickslip phenomena of low metallic friction materials at high temperature induced high torque oscillations during drag test. During this experiment two different braking control modes (pressure controlled and torque controlled modes) were compared. The type of the control mode used for brake test significantly affected the friction characteristics.

• 한국과학교육학회지
• /
• 제37권3호
• /
• pp.523-537
• /
• 2017

본 연구의 목적은 초등학생들이 열 현상과 관련된 문제를 해결하는 과정에서 나타나는 직관적 사고의 특징을 '발현의 맥락'과 '직관적 사고와 논리적 사고의 관계'를 중심으로 살펴보는 것이다. 이를 위해 초등학교 5학년 학생 9명에게 열 현상과 관련된 문제를 제시하고, 학생들의 문제해결 과정에서 나타나는 사고를 살펴보았다. 특히 본 연구에서는 학생의 설명 기저에 있는 사고의 맥락을 총체적으로 탐색 할 수 있는 임상면담 기법을 활용하여 학생의 사고 과정을 분석하였다. 분석 결과, 직관적 사고의 특징을 '발현 과정의 특징'과 '발현의 촉진' 측면에서 발견할 수 있었다. 첫째, 직관적 사고의 발현 과정에서 학생들은 직관적 사고를 통해 해결방법을 먼저 떠올리고 논리적으로 해결방법을 정당화하였으며, 문제 상황과 관련된 구체물과 추상적 개념을 직관적으로 연결 짓는 모습을 보였다. 또한 초등학생들은 충분하지 않은 정보 속에서 직관적인 예감을 통해 문제를 해결해갔다. 둘째, 직관적 사고는 의도적인 사고 전략을 통해 발현되는 경우도 있었다. 이러한 전략으로 그림으로 그려보기, 다른 관점이나 반대로 생각해 보기, 방법을 통합하기가 발견되었다. 이와 같은 문제해결 과정에서의 직관적 사고에 대한 본 연구결과는 그동안 주목받지 못하였던 학생들의 직관적 사고를 드러내어 탐색하고, 학생들의 문제해결 과정을 새로운 관점에서 조력할 수 있는 방안을 제안할 수 있다는 점에서 의미가 있겠다. 또한 본 연구의 결과를 통해 문제해결 상황에서의 과학학습에서 나타난 시사점에 대하여 논의하였다.

• Journal of Mechanical Science and Technology
• /
• 제19권5호
• /
• pp.1206-1215
• /
• 2005

The thermal stratification phenomena, frequently occurring in the component of nuclear power plant system such as pressurizer surge line, steam generator inlet nozzle, safety injection system (SIS), and chemical and volume control system (CVCS), can cause through-wall cracks, thermal fatigue, unexpected piping displacement and dislocation, and pipe support damage. The phenomenon is one of the unaccounted load in the design stage. However, the load have been found to be serious as nuclear power plant operation experience accumulates. In particular, the thermal stratification by the turbulent penetration or valve leak in the SIS and SCS pipe line can lead these safety systems to failure by the thermal fatigue. Therefore in this study an 1/10 scaledowned experimental rig had been designed and installed. And a series of experimental works had been executed to measure the temperature distribution (thermal stratification) in these systems by the turbulent penetration, valve leak, and heat transfer through valve. The results provide very valuable informations such as turbulent penetration depth, the possibility of thermal stratification by the heat transfer through valve, etc. Also the results are expected to be useful to understand the thermal stratification in these systems, establish the thermal strati­fication criteria and validate the calculation results by CFD Codes such as Fluent, Phenix, CFX.

• 대한기계학회논문집B
• /
• 제28권8호
• /
• pp.968-975
• /
• 2004

Nanofluids have anomalously high thermal conductivities at very low fraction, strongly temperature-dependent and size-dependent conductivities, and three-fold higher critical heat flux than that of base fluids. Traditional conductivity theories such as the Maxwell or other macroscale approaches cannot explain why nanofluids have these intriguing features. So in this paper, we devise a theoretical model that accounts for the fundamental role of dynamic nanoparticles in nanofluids. The proposed model not only captures the concentration and temperature-dependent conductivity, but also predicts strongly size-dependent conductivity. Furthermore, we physically explain the new phenomena for nanofluids. In addition, based on a proposed model, the effects of various parameters such as the ratio of thermal conductivity of nanofluids to that of a base fluid, volume fraction, nanoparticle size, and temperature on the thermal conductivities of nanofluids are investigated.

• 대한기계학회논문집B
• /
• 제23권9호
• /
• pp.1201-1212
• /
• 1999

A numerical analysis is performed to predict the thermal response and ablation rate for charring or non-charring material which is designed to be used as thermal protection system (TPS). The numerical program composed of in-depth energy balance equation and the aerotherm chemical equilibrium (ACE) program. The ACE program calculates various thermochemical state from ablation products. The developed numerical program is verified by comparing the reported results from literature. The sensitivity tests for input parameters are performed. The thermal behavior of ablating material is mainly affected by density of ablating material, convective heat transfer coefficient and recovery enthalpy of flow field.

• 한국정밀공학회:학술대회논문집
• /
• 한국정밀공학회 1996년도 춘계학술대회 논문집
• /
• pp.635-640
• /
• 1996

A thermo-elastic system in the production machine has highly nonlinear dynamic characteristics. In general, the finite element method is utilized for accurate analysis. However, it requires large computing time. Thus, thermo-elastic systems are usuallymodeled as electric and fluid system using lumped para,eter. In this paper. we propose the bondgraph model and transient simulation methodology of thermo-elastic system in consideration of various boundary and joint contact conditions. Consequently, the proposed method ensures a possibility of its on-line compensation about undesirable phenomena by using real time estimate process and electronic cooling device for thermal appropriate behavior. Thermo-elastic model consisting of bush and shaft including contact condition is presented.

• The Korean Journal of Ceramics
• /
• 제5권4호
• /
• pp.414-418
• /
• 1999

The synthesis of polycrystalline $Al_2TiO_5$ ceramics with low thermal expansion by fusion in an electric arc furnance was investigated. The thermal expansion curves of $Al_2TiO_5$ ceramics were lowered because of microcracks caused by the strong thermal expansion anisotropy of the crystal axes and were accompanied by hysteresis curves. These phenomena are explained by the opening and closing of microcracks. The difference in microcracking temperatures of dilatometric cooling curves in the range of 400~$620^{\circ}C$ is caused by the difference in sintering temperature, grain size and stabilization status.